1. Field of the Invention
The present invention generally relates to free-space optical (FSO) communications systems, and, more specifically, to a method and apparatus for improving transmitted signal power and link distance between FSO terminals.
2. Background Information
With the increasing popularity of wide area networks (WANs), such as the Internet and/or the World Wide Web, network growth and traffic has exploded in recent years. Network users continue to demand faster networks and more access for both businesses and consumers. As network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to present day hardwired or fiber network solutions is the use of wireless optical communications. Wireless optical communications utilize point-to-point communications through free-space and therefore do not require the routing of cables or optical fibers between locations. Thus, wireless optical communications are also known as free-space or atmospheric optical communications. For instance, in a free-space optical communication system, a beam of light is directed through free-space from a transmitter at a first location to a receiver at a second location. Data or information is encoded into the beam of light, and therefore, the information is transmitted through free-space from the first location to the second location.
Transmission of optical signals through free space poses many challenges. Notably, atmospheric conditions can greatly degrade signal strength and link distances. When launching a single-mode beam from a free-space optical terminal, atmospheric scintillation and other wavefront distortion cause the beam to break up into chaotic bright and dark spots. The received signal may have a large fluctuation if the collector size is comparable to the size of the bright and dark spots.
One technique that is used to address these problems is to xe2x80x9cscramblexe2x80x9d the optical beam, thereby creating a multitude of randomized signals rather than a single mode signal. Mode scrambling may be performed using various techniques and apparatus that are well-known in the art. For example, mechanical mode scramblers have long been used to generate a multimode optical signal. A single mode optical signal is launched from a single mode optical fiber into a multimode optical fiber. The multimode optical fiber is placed in the mode scrambler, which has corrugated surfaces to provide micro-bends in the optical fiber and redistribute energy into all the modes in the multimode optical fiber, resulting in the desired overfilled launch condition. The mechanical mode scrambler physically bends the optical fiber such that the angle of reflection between the optical signal and the core/cladding interface will be altered as the single mode optical signal passes through the portion of the optical fiber being bent. In this way, the single mode launch optical signal will be coupled into many more modes to approximate an overfilled power distribution in the multimode optical fiber. One such mechanical mode scrambler is the FM-1 Mode Scrambler available from Newport Corporation in Irvine, Calif.
Despite the advantages, this type of mode scrambler imposes intolerable strain on the optical fiber when physically bending the optical fiber to alter the angle of reflection. The micro-bending stretches one side of the optical fiber and compresses the other. Because most optical fibers are comprised of glass or plastic, any strain on the optical fibers increases the risk that they will break. Tight bends in optical fiber can cause cracks, which can affect the optical signal traveling through the optical fiber, and will eventually lead to breakage of the optical fiber. A broken or cracked optical fiber will not properly transmit an optical signal.
In addition to problems with optical fiber damage, the characteristics of the scrambled signals produced by conventional mode scrambling techniques are less than optimal. Significantly, the power distribution (i.e., relative intensity vs. angle) of the signal may be asymmetrically skewed and/or peaked, and the numerical aperture is only partially filled. These potentially may lead to substantial signal losses, which may result in erroneous and/or lost data. In addition, mechanical mode scrambling tends to be excessively lossy, reducing the efficiency of the fiber connection.
The present invention provides an apparatus and method for generating a mode-scrambled optical signal with a substantially-filled numerical aperture in multimode optical fiber. A laser beam source directs a laser optical signal into one end of a first segment of multimode fiber comprising a graded-index (GI) fiber core using an offset launch condition. The first segment of multimode fiber is operatively coupled into a second segment of multimode fiber comprising a step-index (SI) fiber. As the laser optical signal passes through the first and second segments of multimode fiber, the optical signal is converted into a mode-scrambled optical signal having a substantially-filled numerical aperture. In one embodiment, the free end of the first segment of multimode fiber is angled at an acute angle relative to the propagation direction of the laser optical signal. In one embodiment, a portion of the second segment of multimode fiber is configured in a series of alternating loops, which causes the outer portion of the numerical aperture to be more completely filled.